The emerging field of “planetary defense” strode into the global spotlight on Tuesday with the first annual Asteroid Day. Coinciding with the 107th anniversary of the Tunguska meteor that struck Siberia, the global movement aims to raise awareness of the risks of future asteroid impacts on Earth.
While most large near-Earth objects (NEOs) are being tracked by NASA and other space agencies, many smaller ones are not — yet — for lack of funding. But even small asteroids (a few hundred meters wide) would cause catastrophic damage on impact, equivalent to many nuclear explosions. The small asteroid (perhaps about 20 meters across) that exploded in an airburst over Chelyabinsk, Russia in February 2013 unleashed energy equal to about 500 kilotons of TNT, some 20 or 30 times the energy released in the Hiroshima atomic explosion. The enormous resulting shock wave shattered glass in the town’s buildings, injuring nearly 1,500 people.
Astronomy magazine points out that statistically, the likelihood of an impact decreases as size and severity increases. A 50-meter asteroid will strike Earth on average once every 2,000 years, causing local scale devastation as it hits with 10 megatons of energy. And once every 100 million years, on average, a 10-kilometer asteroid will strike Earth, unleashing 100 million megatons of energy and causing a mass extinction, like that of the dinosaurs 65 million years ago.
Paul Chodas, of NASA’s Jet Propulsion Laboratory, estimates that planetary scientists know of about 90% of NEOs larger than 1 kilometer in width. They have probably discovered over 50% of those a few hundred meters across, and roughly 15% of those between 100 and 300 meters. And planetary scientists only know of 1% or less of the far more numerous smaller NEOs, those of a few dozen meters or less.
So part of Asteroid Day’s agenda is to get people to sign their petition for increasing the discovery and tracking of near-earth asteroids 100-fold. “There are a million asteroids in our solar system that have the potential to strike Earth and destroy a city, yet we have discovered less than 10,000 — just one percent — of them. We have the technology to change that situation,” it states.
The further advanced notice we have of an incoming asteroid, the better our chances will be of deflecting it (the American Museum of Natural History released a video on exactly this topic to mark Asteroid Day), or at least preparing for impact. Encouragingly, this is no longer just Hollywood sci-fi, but now international government-level crisis-response planning.